v3_core: Teach pat_alias/2 to eliminate duplicated variables

Duplicated variables as aliases in patterns, such as:

  f({_,_}=Dup=Dup) -> ...

will work, but produce sub-optimal code similar to:

  f({_,_}=Dup=NewVar) when Dup =:= NewVar -> ...

with one extra guard test for each duplicated variable.

Rewrite pat_alias/2 to eliminate all duplicated variables. While
we are at it, also simplify handling of tuples, conses, and literals
by using the data functions in the cerl module.

1 files changed, 13 insertions, 0 deletions

diff --git a/lib/compiler/test/match_SUITE.erl b/lib/compiler/test/match_SUITE.erl
index 7522ee985f..9aec0b3d4e 100644
--- a/lib/compiler/test/match_SUITE.erl
+++ b/lib/compiler/test/match_SUITE.erl
@@ -141,6 +141,13 @@ aliases(Config) when is_list(Config) ->
     ?line {a,b} = list_alias2([a,b]),
     ?line {a,b} = list_alias3([a,b]),
 
+    %% Non-matching aliases.
+    none = mixed_aliases(<<42>>),
+    none = mixed_aliases([b]),
+    none = mixed_aliases([d]),
+    none = mixed_aliases({a,42}),
+    none = mixed_aliases(42),
+
     ok.
 
 str_alias(V) ->
@@ -244,6 +251,12 @@ list_alias2([X,Y]=[a,b]) ->
 list_alias3([X,b]=[a,Y]) ->
     {X,Y}.
 
+mixed_aliases(<<X:8>> = x) -> {a,X};
+mixed_aliases([b] = <<X:8>>) -> {b,X};
+mixed_aliases(<<X:8>> = {a,X}) -> {c,X};
+mixed_aliases([X] = <<X:8>>) -> {d,X};
+mixed_aliases(_) -> none.
+
 %% OTP-7018.
 
 match_in_call(Config) when is_list(Config) ->